(1) Field of the Invention
This invention relates to pressure sensors and more particularly to pressure sensors formed by etching trenches in a substrate, undercutting the sidewalls of the trenches, filling the gaps between the trenches with dielectric, and forming piezoresistors in the region of the trenches and filled gaps.
(2) Description of the Related Art
Pressure sensors are widely used in a number of applications. Pressure sensors have been formed by depositing a layer of polysilicon on a sacrificial layer. Piezoresistors art then formed in the layer of polysilicon layer and the sacrificial layer is etched away leaving the layer of polysilicon as a flexible diaphragm. The resistance of the piezoresistors changes as the diaphragm is deflected by pressure changes. Pressure senors have also been formed by depositing piezoresistors on single crystal silicon and etching away the silicon from the back side of the wafer to form a flexible diaphragm.
U.S. Pat. No. 5,531,121 to Sparks et al. describes a method of forming a pressure sensor by forming an N+ region in the surface of a silicon substrate. A silicon layer, such as epitaxial silicon, is then grown from the surface. One or more trenches are then formed into the N+ region and the N+ region is selectively etched away forming a cavity beneath the surface the surface of the silicon layer. The flexible surface over the cavity is then used to form a pressure sensor.
U.S. Pat. No. 5,242,863 to Xiang-Zheng et al. describes a method of forming a pressure sensor by forming a buried low resistance N type layer under a predetermined diaphragm region of the substrate. One or more trenches which extend to the low resistance N type region are then formed at the edges of the diaphragm region. The low resistance N type material is then converted to porous silicon by anodization of silicon in a concentrated hydrofluoric acid solution. The porous silicon is then etched away leaving the diaphragm region over a cavity. The diaphragm is then flexible and can be used to form a pressure sensor.
U.S. Pat. No. 4,766,666 to Sugiyama et al. describes a method of forming a pressure sensor which uses an insulating diaphragm film formed of an etch resistant material on the surface of a substrate. The diaphragm film has at least one etching hole. A cavity is then etched in the substrate under the diaphragm film using the etching holes in the diaphragm film. The diaphragm film over the cavity then becomes a flexible diaphragm and can be used as a pressure sensor.
U.S. Pat. No. 4,853,348 to Tsubouchi et al. describes the formation of a cavity in a substrate in order to form a capacitor having increased capacitance.
This invention describes a method of forming a flexible diaphragm using different methods which can be used as a pressure sensor. The pressure sensor formed is protected from damage due to overpressure and is formed using processes which are compatible with processing of CMOS or other integrated circuit chips.